As wireless communications become more widely used, the number of individual users and communications multiply and, thus, communication system capacity and communication quality become substantial issues. For example, an increase in cellular communication utilization (e.g., cellular telephony, personal communication services (PCS), and the like) results in increased interference experienced with respect to a user's signal of interest due to the signal energy of the different users or systems in the cellular system. Such interference is inevitable because of the large number of users and the finite number of cellular communications cells (cells) and frequencies, time slots, and/or codes (channels) available.
In code division multiple access (CDMA) networks, for example, a number of communication signals are allowed to operate over the same frequency band simultaneously. Each communication unit is assigned a distinct, pseudo-random, chip code which identifies signals associated with the communication unit. The communication units use this chip code to pseudo-randomly spread their transmitted signal over the allotted frequency band. Accordingly, signals may be communicated from each such unit over the same frequency band and a receiver may despread a desired signal associated with a particular communication unit. However, despreading of the desired communication unit's signal results in the receiver not only receiving the energy of this desired signal, but also a portion of the energies of other communication units operating over the same frequency band. Accordingly, as the number of users utilizing a CDMA network increases, interference levels experienced by such users increase.
The quality of service (QOS) of communications and the capacity of the communication network are typically substantially impacted by interference or noise energy. CDMA systems are interference limited in that the number of communication units using the same frequency band, while maintaining an acceptable signal quality, is determined by the total energy level within the frequency band at the receiver. For example, the phenomena known as “pilot pollution” in CDMA systems manifests itself as pilot signal interference associated with reception by a particular subscriber communication unit of pilot signals of a number of base station communication units. For a base station to be received well by a subscriber unit the base station should have a strong pilot signal as received by the subscriber unit. However, the pilot signals of all other base stations received by the subscriber unit provide interference with respect to the other pilot signals. Accordingly, the strength of a particular pilot signal as received by a subscriber unit is not determined from absolute power of the signal, but instead is generally a ratio of signal or carrier to interference (C/I). Similar phenomena is experienced with respect to other communication protocols, e.g., global system for mobile (GSM) systems experience similar effects.
The QOS of communications with respect to communication units may be greatly affected by such interference, even though the power level of communication signals, e.g., pilot or beacon signals, are quite high. Accordingly, outage areas (locations where service is not supported) of cellular networks are often defined in terms of a noise or interference related threshold, such as establishing an acceptable C/I threshold. For example, in a CDMA system an outage area may be defined through use of a threshold such that the pilot Ec/Io (energy per chip of the pilot to the total received interference) is less than a predetermined threshold (e.g., −15 dB). GSM systems implementing frequency hopping schemes experience similar limitations with respect to interference.
Cellular communications systems have typically been conceptualized for analysis and planning purposes as a grid of hexagonal areas (cells) of substantially equal size disposed in a service area. A base transceiver station (BTS) having particular channels assigned thereto conceptually may be disposed in the center of a cell to provide uniform wireless communications throughout the area of the cell. Therefore, a grid of such cells disposed edge to edge in “honeycomb” fashion may be utilized for information with respect to the relative positions of a plurality of BTSs for providing wireless communications throughout a service area.
However, it should be appreciated that the communication coverage associated with a BTS typically varies substantially from the theoretical boundaries of the cell due to cell topology and morphology. For example, topological characteristics (mountains, valleys, etc.) and/or morphological characteristics (large buildings, different building heights, shopping centers, etc.) result in different path losses or other propagation attributes experienced in different azimuthal directions from the BTS. Accordingly, in practice homogeneous signal quality is not provided throughout the area of a cell or throughout the network.
Typically cells have been implemented as omni-trunks, where each cell is able to use each channel in the full 360° azimuth of a BTS, or sectored configurations, such as breaking the cells down into 120° sectors such that each cell channel communicates in the 120° azimuth an associated sector. However, because of the irregular boundaries experienced in actual cell implementations (e.g., path loss variance), a user moving about a cell and even a sector may experience a wide variety of communication conditions, including outage conditions (e.g., Ec/No <−15 dB) or poor quality of service. For example, this user may move only a few degrees in azimuth with respect to a BTS and experience significant signal quality degradation. Accordingly, this user may experience unacceptable communication conditions, such as the aforementioned outage conditions, when noise or interference levels are otherwise generally within acceptable limits for operation within the network.
Both the user's signal of interest, such as a serving pilot signal, and interference associated therewith are typically subject to log-normal shadowing. Accordingly, the communication conditions experienced are dependent on the variance of both.
It can therefore be appreciated that the capacity of the cell may be unnecessarily limited and/or the quality of communications provided thereby may be substandard if the quality of various signals of interest with respect to individual users is not maintained and/or interference energy is not controlled. A need therefore exists in the art for systems and methods which are adapted to provide optimized communications throughout a communication network.